Feeling slower could be related to the fact that your 9x18 is heavier than the 7.5x18 stock. And a higher rotating mass is creating more resistance (needing more power to spin) and if you started wit 7.5 I assume you were on 225er and now on 245-265 ... wider tire more rolling resistance and these are possibly heavier than the stock tire too. And with rotating mass the further away from the axel it is the more if has an effect (and the wider rim and wider tire are far away from the axel).
BUT you are in a 235 with tune and must have a really good calibrated butt dyno to notice these changes. When I drove a Fiat Cinquecento with 55hp the change from 13 inch to 15 inch was very noticeable but that was an underpowered POS
To the original discussion 225/40 and 255/35 would be as close in diameter as possible for a staggered set